Reading Spark Plugs

Introduction:

Next to an O2 (oxygen sensor) or A/F monitor, the only real way to tell what is happening inside the engine with respect to mixture and fuel burn is the spark plugs. This is the time-honored way to tune BTW. First, preparing to read the plugs:

Always clean the plugs (sand blasting, wiring brushing, etc - just get any crud off them) before doing a run. A good lighted magnifier is good too and not terribly expensive (Edmond Scientific carries them among others)

Location:

Load colors the plugs. Running the engine in a speed range with no load is useless. Load is what makes the engine work. If you have a chassis dyno - this will work very nicely. Unfortunately, you probably don't. The other option is the road. Pick a place that has a nice straight section with little or no traffic (and no police in case you get going faster than is posted). A road with a slight grade is nice as it will put a higher load on the engine without going faster (or put more load on the engine - however you wish to look at it). The grade should be as consistent as possible. A good road in my opinion is a 2 mile stretch with a 3-5% grade. Of course what's close is what's important. Once you find a suitable test location always use this location.

Throttle position:

For carburetion / mixture tuning throttle position is normally used as this determines which circuits are being used in the carb. Normal throttle test positions (in a 1:1 or less high gear) are:

NOTE: Motorcycles with Constant Velocity carburetors (like the Yamaha XS1100) this method is meaningless as the vacuum diaphragm determines mid-range mixture. The process can still be used for the 0, 1/8 and WOT positions however.

Making a Throttle Position Indicator:

To determine these positions you can take masking tape and put around the front of the throttle grip. Pick a convenient place and with either white shoe polish (if a black surface) or magic marker (if a light surface) make a solid mark 90 deg or perpendicular to the grip. This is your marker. With the throttle closed make a mark (0). Open the throttle all the way up and make a mark (if you have a motorcycle whose throttle turns more than 1 turn - note that) - WOT (I use a W). You should be able to eyeball the position 1/2 way between 0 and WOT marks. Mark it (1/2 throttle). 0 --> 1/2 = 1/4 - mark it. 0 --> 1/4 = 1/8 - mark it.

Process:

Bring 2-3 sets of plugs. Warm the engine up with a different set of plugs (warm up set). When up to operating temp, switch plugs. Start the bike and run up to the range of interest as quickly as safe and practical. Run at the speed (or throttle position if you're using that method) of interest for several minutes then cut the engine (kill switch) and coast to a stop.

NOTE:Try to keep the engine speed constant and tune based on that.

After you have the plugs looking good test again and this time vary the throttle constantly through the range of interest (or range which your machine is capable on this road) - this means ranges like mid-range or top end with 3000 rpm wide ranges in high gear you may not be able to accelerate / decelerate through those ranges in the length of the road you have. In this case test in sections that are practical - maybe 1000 rpm increments (for mid-range 3000-4000, 4000-5000, 5000-6000).

The varying rpm tests will detect "transition" mixture problems in the range being tested. If you find these, you'll usually have to "split the difference." What this means is that you know at one engine speed your mixture was right. But across this range its either leaner or richer. Instead of adjusting mixture to correct, you can also make a small change to make it better. Retest your constant rpm mixture. It may not have changed hardly any at all! This is a compromise. Of course, you could ignore the constant rpm mixture setting and "tune" for the range too. This isn't wrong. You're shooting for the best running machine, not pretty plugs!

High gear (a gear which is 1:1 in ratio or less. You usually want the gear that is closest to 1.0:1 without being less than 1.0. It will be very hard to accelerate with an overdrive gear (0.9 or less). Get in your "test" gear as quickly as possible. This "high" gear will put the greatest "load" on the engine.

Reading:

Plugs will "color" from the fuel burning around the plug. The tip or center insulator is what changes. Of course new - it is white. When it "colors" it will get darker. If you run and the insulator is white your motor is either 1) really lean or 2) you didn't get enough load or didn't load long enough to "color" the plugs.

Tune to each range. I usually tune idle before going out (no load). You never run the machine at idle so this is appropriate. I also usually do off-idle in front of my house. Again, I rarely run in this range but do want to see what the mixture is (you want to avoid extremes (really lean or really rich). Since you don't normally run here, so long as the mixture is reasonable and the machine accelerates from stop through this range smoothly, service will be satisfactory. Low speed is important, not because you run there so much (though on the XS1100 school zones and 35 mph posted limits fall in this range and running well here, keeps you from having to downshift (losing mileage) because the machine won't run correctly. And throttle snaps from a stop will bog down if the mixture in the low speed range are not correct.

Mid range is by far the most important range on a motorcycle. This is where you run 90% of the time (well most riders). This range is controlled by the needle jet and jet needle primarily with the main jet contributing some at the higher ranges (basically the needle jet is controlling how much air is going to mix with the available fuel from the main jet, the jet needle is a restriction to main jet flow. Hence the reason we say the main jet is a contributor only in the upper ranges. If you're changing the main jet in this range and SEEING changes you have carburetion problems. Get the wrenches out!

WOT: This is the area racers worry about as this is the range they spend most of their time in. The main concern is lean conditions which could harm the engine. During testing, any inability to accelerate -discontinue the testing - kill the engine and check the plugs.

IF THERE IS ANY CHANCE THE MIXTURE COULD BE LEAN, GO UP TO THE NEXT AVAILABLE JET SIZE - THEN ONE PAST THAT AND RETEST. OTHERWISE, ENGINE DAMAGE COULD RESULT.

Once you have tuned all ranges, you may have changed the main jet. Go back and retest the lower ranges. Convince yourself that the main jet is working in it's range and the other jets are working in their's! In high gear, if possible, go from Low speed through WOT rpm. This will almost certainly require lower gears, otherwise you'll be way, way over any posted limits. You want smooth (seamless - unnoticeable) transitions between ranges.

Any hesitation, bucking, surging no matter how slight should be noted.

These are the result of the mixture changing significantly in a short engine speed range. Going from a slightly rich condition (which may have given you a good plug reading) to a slightly lean condition (may have also given a good plug reading) will cause a surge (acceleration is better slightly lean). The other way around will cause a slight hesitation. Don't get frustrated, READING PLUGS WELL IS AN ART! What you gain for your efforts is a superior running machine.

Throttle position - carburetor circuit diagram: (In the near future I will have a link here to a diagram showing mixture circuits versus throttle position)

The diagram shows (this is specifically for Mikuni carburetors) which carburetion circuits are controlling mixture at varying throttle positions. REMEMBER: CONSTANT VELOCITY CARBS CONTROL UPPER LOW THROUGH LOWER WOT BY WAY OF A VACUUM CONTROLLED (HENCE LOAD SENSING) DIAPHRAM.

Therefore throttle position DOES NOT determine jet needle position on these carburetors LOAD DOES!